The electric heaters shaped as test tubes are generally provided with an envelope which is similar for the different uses thereof which are different for the kind of heating element namely heater contained therein. The same can be said for electric heaters shaped as cartridges. Such heaters shaped as test tubes are generally provided with an envelope made of insulating material (normally glass, but also any other insulating material) and have the form of a cylinder with a bottom which is closed by the same material of the cylindrical part thereof, or by a plug. The heating element of the currently used test tubes is generally formed of one or more spirals or windings made of alloys for resistances (for example, NiCr) which are positioned on a core made of ceramic material or mica. Recently, there have appeared in the field of aquarium appliances some kinds of test tubes with a hearing element based on a self-regulating thick film resistance with FTC effect. This technology, which has been used in electronic engineering since several decades in the field of hybrid circuits, consists of producing a circuit by applying one or more conductor inks on a support constituted by a polymeric film, which is then pressed against the test tube inner surfaces However, such technology has the following main drawbacks: high scattering of resistance values of the heating elements of the same lot, due above all to the difficulty of providing a uniform layer thereof, high production costs, the manufacturing process not very clean from the ecological point of view The cartridge heaters are provided with a metallic outer envelope, a cartridge, and made with a cylindrical form, as usual, and also with different prismatic forms.
A most diffused heating element is that constituted by a metallic spiral immersed in the compact magnesium oxide, as in the case of the armored resistances.
A kind of heating element which at the present has a certain diffusion is the PTC (positive temperature coefficient) element with self-regulating characteristics based on ceramic pellets (doped barium titanate). Some constructive products of this kind, having also envelopes made of special materials such as for instance siliconic rubbers filled with large amounts of conductive ceramic powders are provided. The drawbacks of this kind of resistance are: cost and high scattering of resistance values of the single pellets.